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Functional reactive programming for C++
CEFP Summer School, Budapest 2015
Ivan Cukic
KDE University of [email protected] [email protected]
Futures Ranges Streams Further evolution of C++ Epilogue
About me
KDE developmentTalks and teachingFunctional programming enthusiast, but not a purist
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Futures Ranges Streams Further evolution of C++ Epilogue
Disclaimer
Make your code readable. Pretend the next personwho looks at your code is a psychopath and they knowwhere you live.
Philip Wadler
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Futures Ranges Streams Further evolution of C++ Epilogue
Disclaimer
The code snippets are optimized for presentation, it is notproduction-ready code.std namespace is omitted, value arguments used instead of const-refs orforwarding refs, etc.
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Futures Ranges Streams Further evolution of C++ Epilogue
Why C++
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FUTURES
Concurrency
Futures
Futures Ranges Streams Further evolution of C++ Epilogue
Concurrency
ThreadsMultiple processesDistributed systems
Note: "concurrency" will mean that different tasks are executed atoverlapping times.
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Futures Ranges Streams Further evolution of C++ Epilogue
Plain threads are bad
A large fraction of the flaws in software developmentare due to programmers not fully understanding all thepossible states their code may execute in. In amultithreaded environment, the lack of understandingand the resulting problems are greatly amplified,almost to the point of panic if you are paying attention.
John CarmackIn-depth: Functional programming in C++
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Futures Ranges Streams Further evolution of C++ Epilogue
Plain threads are bad
Threads are not composableParallelism can’t be ‘disabled’Difficult to ensure balanced load manually
Hartmut KaiserPlain Threads are the GOTO of Today’s Computing
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Futures Ranges Streams Further evolution of C++ Epilogue
Plain synchronization primitives are bad
You will likely get it wrongS.L.O.W. (starvation, latency, overhead, wait)
Sean ParentBetter Code: Concurrency
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Futures Ranges Streams Further evolution of C++ Epilogue
Amdahl’s Law
1(1−P )+ PN
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Futures Ranges Streams Further evolution of C++ Epilogue
Locks are the main problem
The biggest of all the big problems with recursive mutexes is thatthey encourage you to completely lose track of your locking schemeand scope. This is deadly. Evil. It’s the "thread eater". You hold locksfor the absolutely shortest possible time. Period. Always. If you’recalling something with a lock held simply because you don’t know it’sheld, or because you don’t know whether the callee needs the mutex,then you’re holding it too long. You’re aiming a shotgun at yourapplication and pulling the trigger. You presumably started usingthreads to get concurrency; but you’ve just PREVENTED concurrency.
I’ve often joked that instead of picking up Djikstra’s cuteacronym we should have called the basic synchronizationobject "the bottleneck". Bottlenecks are useful at times,sometimes indispensible – but they’re never GOOD.
David ButenhofRe: recursive mutexes
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Futures Ranges Streams Further evolution of C++ Epilogue
Futures
Futures should be the lowest level concurrency abstractions.
std::futureboost::futureQFutureFolly Future
any continuation - *.then([] . . .)
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Futures Ranges Streams Further evolution of C++ Epilogue
Future
T value = function();
future<T> value = function(); . . .; value.get();)
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Futures Ranges Streams Further evolution of C++ Epilogue
Future
future<T> value = function(); . . .; value.get();
future<T2> value = function().then(continuation);
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Futures Ranges Streams Further evolution of C++ Epilogue
Futures
get_page("http://people.inf.elte.hu/cefp/").then(
[] (auto &&result) {cout << result.headers();
})
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Futures Ranges Streams Further evolution of C++ Epilogue
Futures
get("http://people.inf.elte.hu/cefp/").then(
[] (auto &&result) {cout << result.headers();
for (image: result.image_tags) {image.get().then(
[] (auto &&image_result) {// do something// with image_result
});
}}
)18
Futures Ranges Streams Further evolution of C++ Epilogue
Imperative chaining of futures
result = get("http://people.inf.elte.hu/cefp/"),for_(image = result.image_tags) (
image_result = image.get(),// do something with image_result. . .
)
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Futures Ranges Streams Further evolution of C++ Epilogue
Imperative chaining of futureswhile_(
// Wait until we get a connection.client = ws::server::accept(server),
// Start a detached execution path to process the client.detach_([] {
. . .
serial_(// WebSocket handshakeheader = ws::client::get_header(),server_key = ws::server::create_key(header),ws::client::send_header(client, server_key),
// Sending the initial greeting messagews::client::message_write(client, "Hello, I'm Echo"),
// Connection establishedwhile_(
// getting and echoing the messagemessage = ws::client::message_read(client),ws::client::message_write(client, message)
))
}))
Check out "Monads in chains" from Meeting C++ 201420
RANGES
STL algorithms
Ranges
Futures Ranges Streams Further evolution of C++ Epilogue
Ranges in C++
vector<int> xs;int sum = 0;
for (x: xs) {sum += x;
}
return sum;
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Futures Ranges Streams Further evolution of C++ Epilogue
Ranges in C++
return accumulate(xs.cbegin(), xs.cend(),0);
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Futures Ranges Streams Further evolution of C++ Epilogue
Ranges in C++
return accumulate(xs.cbegin(), xs.cend(),1,_1 * _2);
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Futures Ranges Streams Further evolution of C++ Epilogue
Ranges in C++
How to do an aggregation on a transformed list?
vector<int> xs;int sum = 0;
for (x: xs) {sum += x * x;
}
return sum;
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Futures Ranges Streams Further evolution of C++ Epilogue
Ranges in C++
How to do an aggregation on a transformed list?
sum $ map (λ x → x * x) xs
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Futures Ranges Streams Further evolution of C++ Epilogue
Ranges in C++
How to do an aggregation on a transformed list?
vector<int> temp;
std::transform(xs.cbegin(), xs.cend(),std::back_inserter(temp),_1 * _1);
return std::accumulate(temp.cbegin(),temp.cend());
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Futures Ranges Streams Further evolution of C++ Epilogue
Ranges in C++, boost.range, N4128
How to do an aggregation on a transformed list?
return accumulate(xs | transformed(_1 * _1));
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Futures Ranges Streams Further evolution of C++ Epilogue
Example
transactions| filter(Transactions::price() > 1000)| groupBy(Transactions::customerId())| sort(
Transactions::price().desc() |Transactions::customerName()
);
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Futures Ranges Streams Further evolution of C++ Epilogue
Example boilerplate
namespace Transactions {struct Record {
int customerId;. . .
};
DECL_COLUMN(customerId). . .
}
Column meta-type has all operators implemented, asc(),desc(), etc.
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STREAMS
Futures, again
Futures Ranges Streams Further evolution of C++ Epilogue
Anything you think that you could ever be
for (item: items) {// do something
}
for_each(items, [] (item i) {// do something
});
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Futures Ranges Streams Further evolution of C++ Epilogue
Just passing our time
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Futures Ranges Streams Further evolution of C++ Epilogue
Oh we’ll keep on trying
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Futures Ranges Streams Further evolution of C++ Epilogue
Flow of information
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Futures Ranges Streams Further evolution of C++ Epilogue
Through the eons, and on and on
Web server client connection requestsUser interface eventsDatabase accessI/OAnything and everything
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Futures Ranges Streams Further evolution of C++ Epilogue
Till the end of time
Message passing:continuation!newClientMessageCall-callback:onNewMessage(continuation)Signals-slots:connect(socket, &Socket::newConnection,
receiver, &Receiver::continuation)Any data collection:for_each(xs, continuation)
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Futures Ranges Streams Further evolution of C++ Epilogue
Stream transformation
Streams can only be transformed with algorithms that acceptinput ranges.
map, bind, filter, take, drop, etc.
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Futures Ranges Streams Further evolution of C++ Epilogue
Stream transformation
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Futures Ranges Streams Further evolution of C++ Epilogue
Map / Transform
We have a stream of 2D coordinates (mouse coordinates).
// Projecting on the x-axismouse_position >>=
map(λ point → (point.x, 0))
// Projecting on the y-axismouse_position >>=
map(λ point → (0, point.y))
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Futures Ranges Streams Further evolution of C++ Epilogue
Implementation detail
namespace stream {template <typename Stream, typename Cont>auto operator >>= (Stream &&stream,
Cont &&cont){
return stream.then(cont);}
template <typename Under>auto make_stream(Under &&under);
}
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Futures Ranges Streams Further evolution of C++ Epilogue
Map
template <typename Func, typename Cont>struct map_cont {
map_cont(Func f, Cont c) : f(f), c(c) { }
template <typename InType>void operator () (const InType &in) {
c(f(in));}
Func f;Cont c;
};
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Futures Ranges Streams Further evolution of C++ Epilogue
Fork (or parallel), tee
tee(print) >>=fork(
receiver1,receiver2
)
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Futures Ranges Streams Further evolution of C++ Epilogue
Fork (or parallel), tee
template <typename ... Conts>struct fork_impl;
template <typename Cont, typename ... Conts>struct fork_impl<Cont, Conts...>: fork_impl<Conts...>{
using parent_type = fork_impl<Conts...>;
fork_impl(Cont c, Conts... cs): parent_type(cs...), c(c)
{ }
template <typename InType>void operator() (const InType &in) {
c(in);parent_type::operator()(in);
}
Cont c;};
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Futures Ranges Streams Further evolution of C++ Epilogue
Stateful function objects
class gravity_object {public:
gravity_object() { }
template <typename Cont>void then(Cont &&c) { _f = std::forward<Cont>(c); }
QPointF operator() (const QPointF &new_point) {m_point.setX(m_point.x() * .99 + new_point.x() * .01);m_point.setY(m_point.y() * .99 + new_point.y() * .01);return m_point;
}
private:std::function<void(QPointF)> _f;QPointF m_point;
};
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Futures Ranges Streams Further evolution of C++ Epilogue
Stateful function objects
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Futures Ranges Streams Further evolution of C++ Epilogue
Can stateful function objects be pure?
Like actors changing behaviourOr, treating the function object like its argument is thepast part of the stream (a finite list of elements)
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Futures Ranges Streams Further evolution of C++ Epilogue
Filter
bool pointFilter(const QPointF &point) {return int(point.y()) % 100 == 0;
}
events >>=filter(predicate) >>=
. . .
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Futures Ranges Streams Further evolution of C++ Epilogue
Flat map
template <typename Func, typename Cont>struct flatmap_cont {
flatmap_cont(Func f, Cont c): f(f), c(c)
{}
template <typename InType>void operator () (const InType &in) {
boost::for_each(f(in), c);}
Func f;Cont c;
};49
Futures Ranges Streams Further evolution of C++ Epilogue
Flat mapclass more_precision {public:
more_precision() { }
template <typename Cont>void then(Cont &&c) { _f = std::forward<Cont>(c); }
std::vector<QPointF> operator() (const QPointF &new_point) {std::vector<QPointF> result;
int stepX = (m_previous_point.x() < new_point.x()) ? 1 : -1;for (int i = (int)m_previous_point.x(); i != (int)new_point.x(); i += stepX) {
result.emplace_back(i, m_previous_point.y());}
int stepY = (m_previous_point.y() < new_point.y()) ? 1 : -1;for (int i = (int)m_previous_point.y(); i != (int)new_point.y(); i += stepY) {
result.emplace_back(new_point.x(), i);}
m_previous_point = new_point;return result;
}
private:std::function<void(QPointF)> _f;QPointF m_previous_point;
};
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FURTHER EVOLUTION OF C++
Ranges
Await
Futures Ranges Streams Further evolution of C++ Epilogue
Ranges
Some call it STL 2.o, provides separate views and actionsFilter a container using a predicate and transform it.
std::vector<int> vi{1,2,3,4,5,6,7,8,9,10};auto rng = vi | view::remove_if([](int i){return i % 2 == 1;})
| view::transform([](int i){return std::to_string(i);});
Generate an infinite list of integers starting at 1, square them, take the first 10, and sum them:
int sum = accumulate(view::ints(1)| view::transform([](int i){return i*i;})| view::take(10), 0);
Generate a sequence on the fly with a range comprehension and initialize a vector with it:
std::vector<int> vi =view::for_each(view::ints(1,10), [](int i){
return yield_from(view::repeat(i,i));});
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Futures Ranges Streams Further evolution of C++ Epilogue
Futures
get("http://people.inf.elte.hu/cefp/").then(
[] (auto &&result) {for (image: result.image_tags) {
image.get().then([] (auto &&image_result) {
. . .}
);}
})
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Futures Ranges Streams Further evolution of C++ Epilogue
Imperative chaining of futures
result = get(. . .),
for_(image = result.image_tags) (image_result = image.get(),// do something with image_result. . .
)
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Futures Ranges Streams Further evolution of C++ Epilogue
Monadic await
result = await get(. . .);
for (image: result.image_tags) (image_result = await image.get();// do something with image_result. . .
)
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Futures Ranges Streams Further evolution of C++ Epilogue
Monadic await
await expression is equivalent to:
{auto && tmp = <expr>;if (!await_ready(tmp)) {
await_suspend(tmp, continuation);
}return await_resume(tmp);
}
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Futures Ranges Streams Further evolution of C++ Epilogue
Answers? Questions! Questions? Answers!
Kudos:
Friends at KDE, Dr Sasa Malkov, basysKom
Worth reading and watching:
Iterators Must Go, Andrei AlexandrescuValue Semantics and Range Algorithms, Chandler CarruthSystematic Error Handling in C++, Andrei AlexandrescuAwait 2.o, Gor NishanovRanges proposal, Eric NieblerReactive manifesto, Books on Erlang or Scala/Akka
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